Wound film capacitor

ABSTRACT

Wound film capacitor and method of making wherein at least two layers of shrinkable thermoplastic dielectric are sandwiched between two layers of metallized or, preferably, foil, electrodes. A pair of lead wires are then positioned against the outer surfaces of the foils and rotated together in the manner of mandrels to wind the film and foil layers into a capacitor body. Flat, or otherwise deformed portions formed on a short portion of one end of each lead wire which is outside of the capacitor during winding are then drawn into the center of the capacitor by pulling on the other end of the wires. The capacitor body is then heated to shrink the dielectric and thereby cause the leads to be held in extremely firm engagement with the foil layers. Since the foils encircle at least 270* of the periphery of the lead wires, a very firm, low resistance, pressure bond is made which renders the capacitor able to withstand substantial amounts of heat applied during a soldering operation. The elimination of mandrel holes allows the capacitor to be extremely compact and usable for many applications without the addition of additional thicknesses of sealing materials.

United States Patent Rayburn [451 Jan. 25, 1972 [541 WOUND FILMCAPACITOR Charles C. Rayburn, Falls Church, Va.

[73] Assignee: Illinois Tool Works Inc., Chicago, 111.

[22] Filed: Mar. 5, 1970 [21] Appl. No.: 16,834

[72] Inventor:

Primary Examiner-Thomas J. Kozma Assislant Examiner A. T. GrimleyAttorney-Robert W. Beart, Michael Kovac, Barry L. Clark and Jack R.Halvorsen [57] I ABSTRACT Wound film capacitor and method of makingwherein at least two layers of shrinkable thermoplastic dielectric aresandwiched between two layers of metallized or, preferably, foil,electrodes. A pair of lead wires are then positioned against the outersurfaces of the foils and rotated together in the manner of mandrels towind the film and foil layers into a capacitor body. Flat, or otherwisedeformed portions formed on a short portion of one end of each lead.wire which is outside of the capacitor during winding are then drawninto the center of the capacitor by pulling on the other end of thewires. The capacitor body is then heated to shrink the dielectric andthereby cause the leads to be held in extremely firm engagement with thefoil layers. Since the foils encircle at least 270 of the periphery ofthe lead wires, a very firm, low resistance, pressure bond is made whichrenders the capacitor able to withstand substantial amounts of heatapplied during a soldering operation. The elimination of mandrel holesallows the capacitor to be extremely compact and usable for manyapplications without the addition of additional thicknesses of sealingmaterials.

6 Claims, 16 Drawing Figures PATENTEU JAMES 1912 H913 INVENTOR.

Char/es 6. Rayburn BYQ His All'ys BACKGROUND Film-foil capacitors arequite widely used because they ofier leads have been applied to thesecapacitors after theyare wound, either by soldering them to the ends orby heating them to embed them in the plastic as described and claimed inmy U.S. Pat. No. 3,040,415. Attempts have been made in the past toanchor leads to foil strips, such as by soldering, and then to wind thecapacitor'about the leadsand foil strips as exemplified by U.S. Pat. No.2,579,462 and No. 3,229,174, and German Pat. No. 886,634. Attemptshavealso been made to wind a capacitor about a pair of wires, one ofwhich could become one of its lead wires with the other being withdrawnafter winding, as exemplified by U.S. Pat. No. 2,790,607. An additionalpatent, U.S. Pat. No. 1.960944, shows winding a capacitor about a pairof cylindrical insulating tubes with lead wires being inserted throughthe tubes after winding. Each of these prior art constructions, as wellas all other constructions of which I am aware, is limited in itscapability of providing one or more of the desirable properties of:compactness, lead strength, economy and simplicity of manufacture, anextended .value and voltage range, adaptability to various leadconfigurations, low contact resistance, a solid core to stabilizeperfonnance characteristics and eliminate the tendency to unwind oftenfound in'mandrel-wound capacitors, and a large intimate contact areabetween the foils and the lead wires to conduct and dissipateheat-especially the large quantities of heat often developed as thecapacitor is soldered into itscircuit application.

SUMMARY layers or strips, the outside surfaces of the foil layers beingengaged by a pair of axially extending lead wires. Thelead wires haveflats or other deformations thereon which lock them to the foils afterwinding and heat shrinking the dielectric and prevent their beingrotated in or being pulled out of the capacitor. Since it has been foundthat the foil and dielectric strips tend to wrinkle when wound about alead of nonuniform cross section, the capacitor is wound on smoothportions of the lead wires and the deformed lead portions are pulledaxially inside the capacitor after the winding has been finished and thewraps sealed together, but prior to the heat shrinking operation.

My improved capacitor utilizes the heat shrinkage characteristics ofbiaxially oriented films. Such films include polyester, polypropylene,polystyrene, polycarbonate and combinations thereof. A variation inparticular performance characteristics such as the dissipation factorand the temperature-capacitance relation, may be obtained by using aparticular film or combination of films having the characteristicdesired.

The capacitor has the advantage that relatively large, rigid leads areused, e.g., No. or No. 22 AWG. The leads can be formed in either axial,radial or standup styles.

The general extent of the value-voltage range of the capacitor is fromabout 100 pf.600 v. to 0.1 mfd.200 v. although it is of course possibleto extend therange beyond these values.

Since the capacitor includes no mandrel hole to occupy space and sinceit is, for most purposes, able to be used in its own thinself-encasement, it is obvious that it should provide the smallestpossible film-foil capacitor obtainable.

Although the capacitor system is designed for self-encasement, a varietyof other coating systems are usable as permitted within the style andtemperature limitations of the capacitor materials. For example, theunits may beepoxy dipped, phenolic dipped and waxed, molded, wrap andfilled, potted or canned.

Since the lead wires serve .as the winding mandrel and remain in placeafter winding, there is no possibility of the capacitor graduallyunwinding into the mandrel hole as is often possible in conventionalwinding systems using a retractable mandrel. The solid core of thecapacitor produced by the lead wires is thus a basis for stableperformance characteristics. Since the lead wires are mounted at one endof the foil strips, the capacitor will have greater self-inductance thanthe extended foil formof construction wherein the leads are soldered orotherwise in contact with a substantial number of turns of thecapacitor.

Unlike conventional heat shrunk tab type construction, such asexemplified by U.S. Pat. No. 2,735,970, the lead wires have pressurecontact over the entire foil width and for approximately 270" aroundtheir peripheries. This large amount of intimate pressure contact notonly assures low contact resistance, butalso permits the foils and leadwires to conduct and dissipate heat within the capacitor section andthus protect the dielectric from overheating in localized hightemperature areas as the capacitor is soldered into its circuitapplication. By the provision of deformations such as fiatted regions onthe leadscentrally of the capacitor section, the leads are stronglylocked against translation and rotation. In one lead strength testperformed on a capacitor made in accordance with this invention, a 16pound pull was necessary to pull out a lead wire made of wire having adiameter of 0.03l inches which was flattened so as to be reduced incross section to 0.010 inches in the center of the'capacitor. In anothercapacitor having a lead wire of the same diameter, it took 8 pounds topull out a lead wire which had been reduced in cross-sectional thicknessto 0.015 inches.

My improved capacitor utilizes at least two thicknesses of dielectricbetween the electrode foils and the lead wires. This double filmthickness principally serves to maintain the high dielectric strengthand insulation resistance in this critical mandrel region wherein filmdamage is more likely to occur than in other portions of the capacitorsection. The double films, also, however, can tolerate a highertemperature during lead soldering and thus help prevent lead solderingfrom degrading the dielectric strength of the capacitor. In addition,the presence of a double film between the electrodes and the first turnof the capacitor section reduces the capacitance per unit length ofelectrode in this region. This factor permits a better yield of lowvalue capacitors by allowing the use of longer electrode foils.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 5 is a top plan view of thecapacitor immediately after a winding operation;

FIG. 6 is a top plan view similar to FIG. 5 wherein each of the leadwires has been deformed over a short section of its length;

FIG. 7 is an end sectional view of the deformed portion of one of thelead wires in F IG. 6 taken on the line 7-7;

FIG. 8 is a view similar to FIG. 6 illustrating an intermediate positionin the pulling apart of the lead wires to bring the deformed sectionsthereon inside the capacitor;

FIG. 9 is a plan view similar to FIG. 8 showing the relationship of thelead wires to the capacitor after the deformed portions of the leadwires have been pulled completely into the capacitor and the dielectricheat shrunk around them;

FIG. 10 is a diagrammatic end cross-sectional view illustrating thegeneral relationship of the lead wire cross sections and the adjacentcapacitor foil and film wraps after a winding operation and before thedeformed lead sections of FIG. 6 are pulled into the center of thecapacitor;

FIG. 11 is a diagrammatic end cross-sectional view similar to FIG.except that it shows the relationship of the lead wire cross sections tothe adjacent capacitor wraps after the deformed lead portions have beenpulled into the center of the capacitor and the dielectric film shrunkas in FIG. 9;

FIG. 12 is a view similar to FIG. 9 except that it shows a modificationwhere the lead wires are bent into a radial configuration;

FIG. 13 is a view similar to FIG. 6 except that it shows a modificationwherein the wires have been deformed at the same end of the capacitor;

FIG. 14 is a view similar to FIG. 12 showing the leads after they havebeen pulled into the capacitor;

FIG. 15 is a view similar to FIG. 14 but showing the leads after thecapacitor has been heat shrunk and the leads have been bent and cut offto form stand-up leads; and

FIG. 16 is a view similar to FIG. 1 illustrating a modification.

wherein a pair of one-side metallized dielectric strips are used insteadof separate strips of film and foil.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND PROCESS FIGS. 1-9 illustratethe process steps involved in making the improved capacitor 10 of theinvention. The winding of the capacitor is shown in FIGS. 1-4. Theinitial winding step comprises placing at least one pair ofthermoplastic dielectric film strips 12, 14 in sandwiched relationshipbetween a pair of foil strips l6, l8. Depending upon the voltage ratingthat the capacitor must have, the transverse width of the foil stripsmay be of varying amounts less than the width of the dielectric stripsto provide the desired width of insulating margin portions. A pair ofelongated lead wire members 20, 22, are placed on opposite sides of thefilm and foil layers 12, 14, I6, 18 and mounted in a winding head (notshown) which will rotate them in a clockwise direction about a commoncenter between them so as to wind the layers in the manner shown inFIGS. 2 and 3 (which show the relative positions of the various elementsafter one-half and one full revolution respectively). In FIG. 3, it canbe seen that the foil layers 16, 18 will be held in intimateengagement-with their respective lead wires 20, 22 over approximately270 of the wire peripheries. "can be further seen that the foil layers16, 18 are separated from each other at the center of the capacitor by adouble thickness of dielectric material. As previously noted, thisdouble thickness not only protects the dielectric layers against damagein this critical center region of the capacitor, but also increases theamount of heat which may be applied to the lead wires 20, 22 during asubsequent lead'soldering operation without damaging the capacitor.

The winding of the capacitor is continued until the desired lengths ofthe respective foils 16, 18 for the capacitance desired have been woundin and protected by several outer wraps of the plain dielectric strip 12as can be seen in FIG. 4. When sufficient outer wraps of dielectric 12have been applied, the wraps may be sealed to each other in anyconventional manner such as by gluing or the application of adhesivetape or by heat sealing, utilizing a combination heat sealing andcutting bar 28 in the manner set forth in US. Pat. No. 2,950,070.

At the completion of the winding and sealing operation, the capacitorwill appear as shown in FIG. 5 with the lead wires 20, 22 extending fromeither end of the capacitor body member 26. Since the circular leadwires are relatively easily slidable relative to the capacitor body 26,a deformation such as a flattened region 20b, 22b, as seen in FIGS. 6and 7, is formed on each lead wire 20, 22 by means of a relatively largediameter (about 1 inch) cylinder (not shown) which presses the lead wireagainst a flat plate (not shown). After the lead wires have beendeformed, from a thickness d to a thickness d which is preferablyone-third to one-half the thickness d, they are pulled axially relativeto the capacitor body 26 (FIG. 8) so as to bring the deformed portions20b, 22b into contact with the foil layers 16, 18 at the center of thecapacitor body. Since the deformation formed by a cylinder in contactwith a circular wire member provides a smooth transition section betweenthe two cross sections of the wire, the deformed sections 20b, 22b willreadily slip over the surfaces of the foils I6, 18 without injuringthem. Although the type of foil used is not extremely critical, it hasbeen found that tin foil provides somewhat better results than aluminumfoil for the reason that tin foil will accommodate the heat shrinking ofthe dielectric without the puckering which is sometimes produced inaluminum foil. Furthermore, tin foil is very malleable and allows thelead wires to slip quite freely over its surface.

After the deformed portions 20b, 22b of the lead wires have been pulledinto the center of the capacitor body 26, the capacitor is subjected toheat for a limited period of time in order to cause the dielectric filmlayers 12. 14 to shrink and thereby exclude air from between the layersand bring the foil layers 16, 18 into firm intimate pressure engagementwith the lead wires 20, 22. Although the capacitor, after heatshrinking, (FIG. 9) is ready for use without additional sealing, it isof course possible to apply additional sealing materials if desired orto bend the leads 20, 22 in any manner desired such as into the radiallead style shown in FIG. 12.

The nature of the locking engagement between the lead wires 20, 22 andthe most nearly adjacent film and foil layers, indicated collectively at32, can be seen in FIGS. 10 and 11. FIG. 10, which is a sectional viewtaken along the line 10-10 in FIG. 6 shows how the layers 32 of thecapacitor conform to the lead wire surfaces for about 270 of theperiphery of the lead wires 20, 22 when the lead wire cross section isround. In FIG. 11, it can be seen that the degree of peripheral contactof the heat shrunk layers 32 with the flattened lead wires is evengreater than 270. Since the length of the outer periphery of theflattened or otherwise deformed lead wire cross sections 20b, 22b aresubstantially the same or only slightly greater than that of thecircular lead wires 20, 22, the foils l6, 18 will not be damaged as thedeformed portions 20b, 22b of the lead wires are slid into the center ofthe capacitor. A comparison in FIG. 11 of the initial lead wire crosssection shown in dotted lines with the final cross section shown insolid lines, indicates that the thickness of the final capacitor will bereduced from its thickness after winding. It can also be seen that thefinal flattened shape of the lead wires causes them to fill up almost.the entire internal volume of the capacitor and thus renders thecapacitor more stable.

FIGS. 13-15 illustrate a modified'fonn of capacitor having stand-up,rather than axial, leads. This capacitor style is obtained by merelyforming the fiatted portions 22a, 22b on the lead wires at the same endso that when they are pulled into the capacitor body 26', their oppositeends 20', 22 will project in the same direction. After the capacitorbody 26 in FIG.

14 is heated to shrink the dielectric, it will assume the shape shown inFIG. 15 at which point the lead wires 20, 22' may be bent in divergingdirections and cut off as seen in FIG. 15.

Although the slipped lead concept of the invention set forth hereinfinds its greatest utility as applied to film foil capacitors, it wouldalso be of some usefulness as applied to capacitors made of two stripsof one-side metallized dielectric material. Such strips, shown in FIG.16 at 112, 114 as having metallized coatings 116, 118 in contact withlead wires 120, 122, would be wound similarly to the film and foilstrips previously described. However, since the metallized coatings 116,118 are of extreme thinness, the lead wires used therewith would have tobe extremely smooth to avoid causing damage to them as they were slidover the coatings.

Iclaim:

1. A wound film capacitor comprising two layers of electrode and atleast two layers of dielectric film, a transverse portion of each ofsaid electrode and dielectric film layers being positioned between apair of lead wires having axial portions about which the layers arewound, each of said electrode strips having a longitudinally extendingportion thereof wrapped around at least 270 of the periphery of eachlead wire, said lead wires including at least two differentcross-sectional configurations along the length thereof which is incontact with said electrodes, the difierent cross-sectionalconfigurations being connected by a smooth transition section, saiddielectric film being heat shrunk tocause the foil electrode strips tobe tightly held in intimate, direct pressure friction and mechanicalcontactwith said lead wires.

2. The capacitor of claim 1 wherein the two layers of elec-' trode areseparated from each other by at least two layers of dielectric film.

3. The capacitor of claim 1, wherein said layers of dielectric are of athermoplastic film which is shrinkable upon being subjected to heat.

7 heat thereto so as to cause said electrode layers to tightly engagesaid lead wires and prevent said lead wires from being rotated or movedaxially relative to the body of the capacitor.

5. The capacitor of claim 4 wherein said electrodes are strips of metalfoil.

6. The capacitor of claim 5 wherein said metal foil is tin.

' i I t i

2. The capacitor of claim 1 wherein the two layers of electrode areseparated from each other by at least two layers of dielectric film. 3.The capacitor of claim 14, wherein said layers of dielectric are of athermoplastic film material which is shrinkable upon being subjected toheat.
 4. The capacitor of claim 1, wherein said lead wires are ofcircular cross section except in a central region intermediate the sideedges of said electrodes where they are partially flattened, saiddielectric layers being formed of a shrinkable thermoplastic filmmaterial which is shrunk by the application of heat thereto so as Tocause said electrode layers to tightly engage said lead wires andprevent said lead wires from being rotated or moved axially relative tothe body of the capacitor.
 5. The capacitor of claim 4 wherein saidelectrodes are strips of metal foil.
 6. The capacitor of claim 5 whereinsaid metal foil is tin.